Power operated trowels or finishing machines are used to finish the surface of concrete slabs. The typical finishing machine includes a plurality of blades which are mounted on the lower end of a central rotor, and a gasoline engine is operably connected to the rotor and serves to rotate the blades directly against the concrete surface. The typical finishing machine includes a mechanism for varying the pitch of the blades. Each blade is pivoted about a longitudinal horizontal axis and can be pivoted from a generally horizontal position to a tilted position where the blade extends at an angle to the horizontal.
The pitch of all the blades is changed simultaneously and by varying the pitch, the amount of the blade surface contacting the concrete slab is varied, so that the machines weight is carried by a larger or smaller area of the blade.
The pitch of the blades is frequently varied as the operator moves the machine over the concrete slab. For example, when the concrete is fresh or wet, the blade pitch will normally be adjusted so that the blades are generally horizontal. As the concrete hardens, the blade pitch can be progressively increased. Further, when the concrete slab is uneven and contains high and low areas, the operator may wish to rapidly change the pitch as the machine is moved between the high and low areas.
The trailing edges of the blades are spaced laterally from the blade pivot axis and as the blades are tilted from a horizontal to an inclined or tilted position, the machine is correspondingly raised relatively to the concrete slab. As the machine is heavy, weighing several hundred pounds, the pitch changing mechanism should enable the operator to tilt the blades quickly and with minimum effort.
In the past, a threaded pitch control mechanism has been utilized to vary the pitch of the blades. In the typical pitch control mechanism, a rod or shaft is mounted within the handle of the machine, and the lower end of the shaft is threaded to a sleeve which, in turn, is connected to a cable that extends downwardly within the handle and is connected to the pitch control lever. By rotating the shaft through operation of a hand knob, the sleeve will be moved axially within the handle to thereby tilt the blades relative to the surface to be finished. The threaded connection between the shaft and the sleeve in the conventional machine has had a pitch of at least eight threads per inch, thereby requiring eight revolutions of the hand knob to move the sleeve one inch and provide a tilt of approximately 10.degree.. Because of the large number of revolutions required to substantially change the pitch, control mechanisms of this type have been relatively slow and have not satisfied the operator's need for a rapid pitch control. It has not been practical in the past to change the pitch of the threaded connection to a lesser number of threads per inch, because the weight of the machine acting on the tilted blades will tend to back off the threaded connection, so that the blade pitch cannot be maintained.
Published PCT application WO87/90221 discloses a pitch control mechanism for a concrete finishing machine, which utilizes a lever control rather than a threaded connection. In addition, the pitch control device of the aforementioned application incorporates a counterbalancing spring which is connected to the control lever and applies a counterbalancing force to enable the lever to be moved with considerably less force than would otherwise be required.
However, the lever-type blade pitch control has certain disadvantages. The lever mechanism has a series of incremental locked positions, and thus it does not have infinite pitch control, so that the operator does not have precise pitch control as with a screw or threaded type of pitch adjustment.
As a further disadvantage, the force of the counterbalancing spring associated with the lever type of pitch control mechanism is designed for a given weight of machine. If the weight changes, as for example by changing the engine, the counterbalancing force has to be readjusted.
The typical power operated trowel machine is moved or pushed manually across the concrete slab by the operator. With a lever-type of pitch control mechanism, a pushing force applied to the lever may tend to move the machine in a direction which is unwanted by the operator and to prevent this, the operator will normally stop movement of the machine over the slab when the pitch adjustment is made.